1. Field of the Invention
The present invention relates to composite bicomponent fibers having a sheath-core structure. The advantages of the composite bicomponent fiber are achieved principally by the cooperation of the characteristics of the core component, such as high tensile strength and low cost, with the enhanced surface properties of the sheath component, particularly resistance to staining, water, chemicals, and high temperatures, along with low electrical conductivity.
2. Prior Art
Composite bicomponent sheath-core fibers and production processes therefor are known. Typically, nylon fibers, nylon 6, nylon 6,6, or copolymers thereof, are used as a core component (see for example U.S. Pat. No. 5,447,794-Lin). The sheath component is typically a variation of the same material as the core material, as shown by Lin, or a polymer such as a polyester or polyolefin (see Hoyt and Wilson European Patent Application No. 574,772). Composite, bicomponent, sheath-core fibers are generally made by delivery of the two component materials through a common spinnerette or die-plate adapted for forming such composite, bicomponent, sheath-core fibers.
Generally, composite bicomponent sheath-core fibers have been used in the manufacture of non-woven webs, wherein a subsequent heat and pressure treatment to the non-woven web causes point-to-point bonding of the sheath components within the web matrix to enhance strength or other such desirable properties in the finished web or fabric product. Other uses of composite bicomponent sheath-core fibers include the production of smaller denier filaments, using a technology generally referred to as "islands-in-the-sea", to produce velour-like woven fabrics typically used for apparel.
Such technology is typically employed in the production of relatively large diameter, monofilament, composite, bicomponent individual monofilaments are grouped into a multifilament yarn. However, the spinning of a small denier multifilament yarn bundle, e.g. less than 100 denier comprised of many (e.g. ten or more) individual sheath-core continuous filaments, is generally commercially unavailable because of the complexities associated with the process and materials used for the sheath and core components.
In order to successfully spin a small denier multifilament yarn bundle comprised of a plurality of individual, composite, bicomponent, sheath-core fibers, the limitations imposed by the known production processes and the materials used as the core and sheath components must be overcome. The demanding requirements of the final composite yarn would be met by simultaneously extruding two different materials in a common process, which requires a degree of Theological, thermal and viscoelastic similarity between the two materials. Additionally, the complexity of quality extrusion increases as the diameter of the individually extruded composite bicomponent sheath-core fibers decreases. Further, once the extruded filaments exit the spin-plate of the spinnerette or die-plate, the filaments must be drawn, typically employing an annealing process done at high speed and under tension, to align the crystal structure and develop strength in the overall composite.
A similarity in stress/strain behavior of the materials used for the core component and the sheath component is required to avoid premature overstretching and breaking (% elongation) during the drawing process. Additionally, sufficient elongation, and tensile strength (tenacity) must be achieved in the final composite yarn to withstand the physical rigors of weaving. Further, the generally thin sheath component should withstand high abrasion while maintaining its integrity and encapsulation of the core component.
The choice of materials used for the sheath-core components is limited by both the rigors of the manufacturing process and the requirements of the final composite yarn. The prior art includes at least the following combinations of materials for sheath-core fibers:
sheath core polyethylene terephtalate polyethylene (PE) (polyester, PET) PET polypropylene (PP) PP PET nylon 6 nylon 6,6 PET, PP, nylon 6 water soluble components
The rheological and viscoelastic properties of thermoplastic fluoropolymers such as polytrifluoroethylene (PTFE), are very dissimilar to the above listed materials. Consequently few such fluoropolymers have been made as one component fibers, particularly in a multifilament format. For example, PTFE has not been known to be melt processible and has only been described as extruded in a proprietary wet spinning process wherein the PTFE latex is mixed and coextruded with a cellulosic dope.